1. Field of the Invention
The invention relates to a method for detecting the position of a rotor of an electronically commutated electric machine, in particular of an electric motor, in which a zero crossing of a voltage induced in a phase winding of the rotor or stator is used for position detection and in order to detect the zero crossing, the phase winding is briefly switched into a currentless state.
2. Description of the Prior Art
For an electronic commutation of an electric machine having a rotor and a stator, it is necessary to know the position of the rotor of the electric machine. This can be achieved by means of position detection. Position detections of the rotor can be carried out both with and without sensors. The electronically commutated electric machine typically has at least one phase winding, which is composed of at least one winding associated with the rotor or the stator. With the commutation, the winding constitutes an electromagnet that can be switched on and off. The part of the electric machine that does not have the phase winding, i.e. the stator or rotor, preferably has at least one permanent magnet that cooperates with the phase winding and produces a rotation of the rotor. The permanent magnet can also be replaced by an electromagnet. To detect the position of the rotor without a sensor, a zero crossing of an electric voltage, which the permanent magnet induces in the phase winding, is detected in the electric machine. To accomplish this, the induced voltage in the phase winding is measured. To be able to detect the zero crossing, the phase winding must be switched into a currentless state, i.e. it must not be supplied with current from the outside. This is because in the phase winding used for producing a force that drives the rotor, this production of force depends on an efficiency of the sum of all induced voltages and of the phase winding voltage. This means that in a hypothesized physically ideal phase winding, the current inside the phase winding flows in phase with the induced voltage. From this, it follows that the current in the phase winding is superimposed with the induced voltage so that a measurement of the zero crossing is not possible without taking corresponding steps. For this reason, the current in the phase winding is switched off before an expected zero crossing and is switched on again only after the zero crossing.
It is disadvantageous that the switching-off of the phase winding means that for the length of time that the switched-off phase winding remains switched off, no force production can occur and therefore no power can be generated. The electric machine consequently loses power density. Simply reducing the length of time during which the phase winding is switched to the currentless state so as to increase the power density of the electric machine results in an increased probability of an incorrect commutation. This is due to the fact that the subsequent switching-off of the phase winding is calculated based on the rotor speed and an expected speed change. As a result, when the length of time is reduced, only small speed changes can be taken into account since the zero crossing must lie within this time period. For this reason, in electric machines with a small speed change over time, not every zero crossing has to be measured; instead one or more zero crossings can be skipped. Considered over the operating time of the electric machine, this procedure yields a higher power density of the electric machine. But if high speed dynamics of the electric machine, i.e. significant and frequent speed changes, make it necessary to detect the zero crossing very often, then the power density can only be retained by minimizing the length of time during which the phase winding is switched off. For this reason, a method is required that permits the phase winding to be switched off as late as possible before an expected zero crossing and switched back on again quickly after the zero crossing.